There are a large number of patients with cataracts, and when the condition worsens, an operation to insert an intraocular lens (IOL) is needed. In the operation to insert an intraocular lens for cataracts, it is necessary to measure at least the ocular axial length and the cornea curvature radius of the patient. At that time, if the measurement is not taken accurately, there will be a difference in vision between the two eyes after an operation. Preferably, the difference in vision between the two eyes should be as small as possible, and it is necessary to leave it in the range of at least ±1D. Therefore, there is a need to improve the precision of these measurements as much as possible.
Conventionally, to measure the ocular axial length and a curvature radius, there is an ultrasound measurement method as shown in Japanese Patent Laid-Open No. 2010-172538, and an optical measurement method as shown in Japanese Patent Laid-Open No. 2008-188047. As well as the need for anesthesia in the ultrasound method, this technique is very hard on the patient physically, and it requires a great deal of skill on the part of the doctor to perform the operation.
The conventional optical ocular axial length measurement device put to practical use uses a time-domain interference method. In the time-domain device using the ocular axial length measurement, the infrared light beam in the 780-840 nm band is irradiated perpendicularly against the eyeball via an interferometer, the light path difference of the reflected light from the cornea and the reflected light from the retina are detected by the movement of each of the reference mirrors, and the ocular axial length is found by calculating the difference. According to this method, the value of the corneal thickness or the lens thickness, or other thickness can also be theoretically detected.
However, there were problems in this measurement device in that it was highly possible that an error would be generated based on the error signal due to the method for moving the reference mirror of the interferometer mechanically, and due to measuring the ocular axial length from a one-dimensional signal. Further, there was the problem that it was necessary to take, for example, around five measurements and then use the average reading, and each measurement took a long time from around 30 seconds to a minute. In addition, there was the problem that if the measurement took a long time, the patients did not remain still, the measurement conditions were not uniform, precise measurement could not be taken, and more time was needed for a diagnosis.
Furthermore, in the case of severe cataract patients, there are times where the ocular axial length cannot be measured without arriving at the retina, since the infrared measurement light is scattered in the clouded crystalline lens. In such cases, the optical measurement of Japanese Patent Laid-Open No. 2008-188047 is not dependable.
On the other hand, Changho Chong et al., “Large Coherence Length Swept Source for Axial Length Measurement of the Eye,” Applied Optics 48:10 (2009): D145-150 suggests a method to measure the ocular axial length by a two-dimensional optical coherence tomography (OCT) image using the wavelength scanning-type-light source (hereinafter referred to as swept-source OCT (SS-OCT)) in the 1 μm band. Since the wavelength in the 1 μm band is a wavelength longer than 800 nm band, and the tolerance value of the optical power that can go into the eyes is high, the light easily arrives at the retina, and since the sensitivity by the SS-OCT method is 20 dB or more higher than that by the time-domain method, it can be expected that the measurement success rate improves. In addition, the scan rate is faster from several kHz to several tens of kHz, therefore a precise measurement can be taken since there is little influence from the blur caused by the movement of the eyes of the patient.
To calculate the power of the intraocular lens, it is necessary to calculate the curvature radius of the cornea as well as the parameter of length, such as the ocular axial length or the anterior chamber thickness. The measurement of curvature radius of the cornea is called keratometry (sic: corneal warpage). Illumination light is mainly irradiated on the corneal surface and the curvature in the range of a two or three mm diameter is generally calculated from the top of the cornea based on the position of the calescence point of the source of light that moved to the surface.